1. Field of the Invention
This invention relates to a GaInP epitaxial stacking structure, and more specifically to a GaInP epitaxial stacking structure for FETs and a fabrication method thereof, which has electron-supply layers and spacer layers which give high-mobility characteristics, and a high-mobility field effect transistor using this structure.
2. Description of the Prior Art
Schottky junction-type field effect transistors (known as MESFETs) which operate in the microwave region or millimeter wave region include GaInP high electron mobility transistors (known as TEGFETs, MODFETs and the like) which utilize mixed crystals of gallium-indium phosphide (GaAIn1-AP: 0≦A≦1) (see IEEE Trans. Electron Devices, Vol. 37, No. 10 (1990), pp. 2141-2147). GaInP MODFETs can be used as low-noise MESFETs for signal amplification in the microwave region (see IEEE Trans. Electron Devices, Vol. 46, No. 1 (1999), pp. 48-54) and as power MESFETs for transmission applications (see IEEE Trans. Electron Devices, Vol. 44, No. 9 (1997), pp. 1341-1348).
FIG. 1 is a schematic diagram of the cross-sectional structure of a conventional GaInP TEGFET. The substrate 10 used is made of semi-insulating gallium arsenide (chemical formula: GaAs) with a {001} crystal plane as its primary plane. Upon the substrate 10 is deposited a buffer layer 11 consisting of a high-resistance Group III-V compound semiconductor layer. Upon the buffer layer 11 is deposited an electron transporting layer (channel layer) 12 consisting of n-type mixed crystals of gallium-indium arsenide (GaZIn1-ZAs: 0<Z≦1). A spacer layer may be deposited upon the channel layer 12, but particularly in power TEGFETs for transmission applications, an electron-supply layer 13 consisting of mixed crystals of gallium-indium phosphide (GaYIn1-YP: 0<Y≦1) is deposited without an interposed spacer layer. The carrier (electron) density of the electron-supply layer 13 is adjusted by the intentional addition (doping) of silicon (Si) or other n-type impurities which are not readily diffused. Upon the electron-supply layer 13, a contact layer 14 consisting of n-type GaAs or the like is typically provided in order to form the low-contact resistance source electrode 15 and drain electrode 16. In addition, between the source and drain electrodes 15, 16, the contact layer 14 is partially removed to expose a recess structure, and a Schottky junction-type gate electrode 17 is provided, thereby constituting a TEGFET.
The various constituent layers 11-14 which constitute the GaInP epitaxial stacking structure 1A for MODFET application illustrated in FIG. 1, because of their ease of film formation, are conventionally formed by the metal-organic chemical vapor deposition (MOCVD) method (see ibid IEEE Trans. Electron Devices, Vol. 44 (1997)). Among these constituent layers, the electron-supply layer 13 is a functional layer for supplying electrons formed to accumulate as a two-dimensional electron gas (TEG) in the vicinity of the junction interface 12a of the channel layer 12. The electron-supply layer 13 is conventionally formed of gallium-indium phosphide (GaYIn1-YP: 0<Y≦1) doped with silicon (the symbol of element: Si) or other n-type impurities which are not readily diffused (see ibid IEEE Trans. Electron Devices, Vol. 44 (1997)). The carrier density (units: cm−3) of the electron-supply layer 13 is commonly made 1-3×1018 cm−3 or 2×1018 cm−3 in particular. The thickness of the layer is typically set within the range 10 nm to 40 nm. In addition, in a GaInP TEGFET, the n-type electron-supply layer is normally constituted from GaYIn1-YP (0<Y≦1) layers wherein the gallium composition ratio (=Y) is fixed in the layer thickness direction.
In addition, in the structure wherein a spacer layer is deposited upon the channel layer 12, in order to prevent the two-dimensional electron gas from being disturbed due to ionization scattering from the channel layer 12, the spacer layer is a functional layer provided for the spatial isolation of the channel layer 12 and electron-supply layer 13 (see “Physics and Applications of Semiconductor Superlattices,” Physical Society of Japan, ed. (published by Baifukan, Sep. 30, 1986, first edition, fourth printing), pp. 236-240). In a GaInP TEGFET, the spacer layer is typically constituted from undoped GaXIn1-XP (0<X≦1) (see ibid IEEE Trans. Electron Devices, Vol. 44 (1997)). Regardless of the case of GaInP TEGFET, spacer layers are constituted from high-purity undoped layers with a low total amount of impurities, and their layer thickness is typically in the range from 2 nanometers (nm) to 10 nm (see ibid “Physics and Applications of Semiconductor Superlattices,” pp. 18-20).
For example, in a low-noise GaInP TEGFET, the noise-figure (NF) and other major properties vary depending on the electron mobility, so the higher the electron mobility, the lower the NF conveniently becomes. For this reason, in order to cause the electrons supplied from the n-type electron-supply layer 13 to accumulate as a two-dimensional electron gas in the interior regions of the GaZIn1-ZAs (0<Z≦1) in the vicinity of the junction interface with the spacer layer consisting of undoped GaXIn1-XP (0<X≦1), the composition at the junction interface between the channel layer 12 and the spacer layer must change abruptly and exhibit high electron mobility.
In addition, the formation of a buffer layer is typically performed by vapor deposition without varying the starting material species of gallium (element symbol: Ga). Since the admixture of carbon (element symbol: C) acceptors that electrically compensate residual donor components represented by silicon occurs readily, and a high-resistance GaAs layer or AlLGa1-LAs layer is easily obtained in the undoped state (see J. Crystal Growth, 55 (1981), pp. 255-262), trimethyl gallium (chemical formula: (CH3)3Ga) is used as the gallium (Ga) source (see J. Crystal Growth, 55 (1981), pp. 246-254, ibd, pp. 255-262, and PCT application publication No. 10-504685).
In a GaInP TEGFET for low-noise amplification, the noise-figure (NF) and other major properties vary depending on the two-dimensional electron mobility (units: cm2/V·s), so the higher the electron mobility (cm2/V·s), the lower the NF becomes. For this reason, in a low-noise TEGFET, the electron-supply layer which takes the role of supplying electrons must be constituted from GaYIn1-YP (0<Y≦1) which can exhibit a high electron mobility. On the other hand, in a power TEGFET, from the standpoint of causing it to operate with a relatively large source-drain current flowing, a large sheet carrier density (units: cm−2) is required together with the electron mobility. Therefore, electron-supply layer for power TEGFET applications must be constituted from a GaYIn1-YP (0<Y≦1) layer that exhibits a high sheet carrier density.
However, in the conventional electron-supply layer consisting of GaYIn1-YP wherein the gallium composition ratio (=Y) or indium composition ratio (=1−Y) is roughly constant, at a relatively high sheet carrier density, there is a disadvantage in that a high electron mobility cannot be manifested stably. For this reason, in low-noise GaInP TEGFETs for example, a large transconductance (gm) is not obtained, thus obstructing the stable supply of low-noise GaInP TEGFETs with a superior low noise-figure (NF).
A first object of the present invention is to provide a GaInP epitaxial stacking structure containing a GaYIn1-YP (0<Y≦1) electron-supply layer and fabrication method thereof for stably manifesting a high electron mobility in excess of 5000 cm2/V·s at room temperature and at a relatively high sheet carrier density of 1.5×1012 cm−2 or greater and 2.0×1012 cm−2 or less. With this structure, low-noise GaInP high electron mobility transistors with superior transconductance properties and power GaInP TEGFETs with superior power transformation efficiency due to their high source-drain current can be provided.
In addition, in a structure wherein a spacer layer is provided between the channel layer and electron-supply layer, if a GaXIn1-XP spacer layer wherein the indium composition ratio (=1−X) is roughly constant is provided joined to the GaZIn1-ZAs (0<Z≦1) channel layer 12, mutual diffusion occurs between phosphorus (element symbol: P) and arsenic (element symbol: As) in the vicinity of the junction interface 12a, so a problem occurs in that the steep change in composition at the junction interface 12a is worsened.
If the steepness of change in composition at the junction interface 12a is not achieved, a two-dimensional electron gas does not efficiently accumulate in the interior regions of the GaZIn1-ZAs channel layer 12, and the electron mobility drops. The electron mobility particularly affects the transconductance (gm) of GaInP TEGFETs for low-noise amplification, and influences the noise-figure (NF) even more. At a low electron mobility, a high gm is not obtained and therefore, a GaInP TEGFET with a low NF is not obtained.
In addition, it was conventionally common for the spacer layer to be constituted from an undoped GaXIn1-XP (0<X≦1) layer wherein the indium composition ratio is constant. However, the carrier density in the undoped state is roughly 1×1016 cm−3 at the lowest. Since the two-dimensional electron gas accumulates more efficiently by lowering the carrier density of the spacer layer, in order for a high electron mobility to be manifested, the spacer layer must be constituted from a GaXIn1-XP (0<X≦1) layer with an even lower carrier density.
Thus, a second object of the present invention is to provide an epitaxial stacking structure comprising a spacer layer made of GaXIn1-XP (0<X≦1) which can stably manifest an even higher electron mobility and has a low carrier density. With this structure, it is possible to provide a GaInP epitaxial stacking structure with excellent transconductance.
Irrespective of GaInP TEGFETs, the transconductance (gm) and pinch-off characteristics of high electron mobility field effect transistors are known to fluctuate depending on the quality of the buffer layer. For example, in the normal AlGaAs/GaAs lattice-matched TEGFETs and AlGaAs/GaInAs strained-lattice TEGFETs, a high gm and good pinch-off characteristics are obtained, and the buffer layer is formed as a high-resistance layer with a low leakage current.
On the other hand, as described above, in a GaInP TEGFET comprising an electron-supply layer consisting of GaYIn1-YP which is one type of a phosphorus- (element symbol: P) containing Group III-V compound semiconductor, simply making the buffer layer a high-resistance layer has conventionally had the problem wherein a homogenous gm and pinch-off voltage cannot be stably obtained. The present inventors discovered that this instability of properties derives from heterogeneity in the indium composition ratio (=1−Y) of the GaYIn1-YP electron-supply layer due to differences in the gallium (Ga) source utilized in the formation of the buffer layer of a superlattice structure that uses AlGaAs and GaAs in particular as constituent layers.
In addition, in the buffer layers consisting of the conventional constitution such as AlGaAs/GaAs superlattice-structure buffer layers, there are problems regarding the DC properties (static properties) of the transistor in that fluctuation in the source-drain current value under illumination (so-called “photoresponsibility”) (see G. J. Ree, ed., Semi-Insulating III-V Materials, (Shiva Pub. Ltd. (Kent, UK, 1980), pp. 349-352) and “hysteresis” of the source-drain current (see Makoto Kikuchi, Yasuhiro Tarui, eds., “Illustrated Semiconductor Dictionary,” (Nikkan Kogyo Shimbunsha, Jan. 25, 1978), p. 238) and “kinks” easily occur (JP-A-10-247727 and JP-A-10-335350).
Therefore, a third object of the present invention is to provide an epitaxial stacking structure comprising a buffer layer for forming a GaYIn1-YP (0<Y≦1) electron-supply layer that has high resistance suitable for reducing the leakage current and that has a homogeneous indium composition.
In a GaInP TEGFET, the spacer layer is constituted from GaXIn1-XP (0<X≦1) which is an indium-containing Group III-V compound semiconductor, and moreover it is constituted as a thin film. The conventional MOCVD technology has a problem in that thin-film spacer layers with a homogenous indium composition ratio (=1−X) cannot be stably obtained.
For this reason, conventional GaInP high electron mobility field effect transistors which use as the spacer layer a GaXIn1-XP (0<X≦1) layer wherein the indium composition ratio is not sufficiently homogenous cannot maintain a homogenous band offset with the channel layer due to “fluctuation” in the indium composition ratio within the spacer layer, and for this reason, achieving a homogenous transconductance (gm) and pinch-off voltage was difficult.
Therefore, a fourth object of the present invention is to provide an epitaxial stacking structure for TEGFET applications that has a GaXIn1-XP (0<X≦1) spacer layer with a superior homogeneity in its indium composition. With this structure, it is possible to provide a GaInP high electron mobility transistor with superior homogeneity in its pinch-off voltage and other properties.